What Does that Mean? – Part 1

Well, I’ve been blathering on about astronomy for a while now and using terminology familiar to astronomy buffs. If you’re not into astronomy maybe the jargon I use just confuses, and that’s not unusual. I’m mystified by the terminology used in the film industry – for example, Key Grip, Gaffer, Foley – what?

So, this and the next article are dedicated to explaining some of the most often used astronomy terms, especially those that might cause confusion and head scratching.

Apparent Field of View: When looking into an eyepiece, it’s size of the image circle you see. Usually expressed in degrees.

Asterism: A prominent star pattern but not a constellation. Example: The Big Dipper.

Astronomical Unit (AU): Distance from Earth to the Sun, 93,000,000 miles/150,000,000 kilometers.

Baily’s Beads: Viewing with a solar filtered telescope. Just before solar eclipse totality, rays of light shining between the Moon’s mountains look like a curved string of bright beads.

Celestial Pole: The imaginary axis Earth rotates on shoots in both directions, north and south. Polaris, the North Star is closest to the north celestial pole. There currently is no very close star for the southern imaginary axis and south celestial pole.

Circumpolar: A constellation or object that is close enough to a celestial pole, so it never sets.

Conjunction:  The Moon, or a planet appears very close in the sky to a planet or star.

Dark Adaptation:  After about 30 minutes in the dark your eyes iris’s open to their maximum – called night vision. Just a momentary exposure to white light will ruin dark adaptation.

Elongation: The planets Mercury and Venus orbits are inside Earth’s orbit so their angular distance from the Sun from our perspective is called their elongation. The greater the elongation, the higher in the sky they appear.

Ephemeris: A table that shows where and at what time a planet, or other body moving relative to stars will be.

Field of View: The actual area you are seeing when looking through binoculars or a telescope.

f/number (or f ratio): Divide the mirror or objective lens diameter by its focal length. A 100mm diameter lens or mirror with a 400mm focal length is f/4. Used to estimate exposure times.

Gibbous: The Moon is more than half lit, but less than a full Moon. Latin – gibbus, meaning hump.

Inclination: Difference between an orbit and a reference point. Our Earth’s orbit is inclined 7.2 degrees vs. the Sun’s equator.

Light Pollution: Glow in the night sky caused by improper use of lighting.

Light Year: The distance light travels in one year – about 6 trillion miles!

Magnitude: A number indicating the brightness of a celestial object. A larger number = dimmer.

Meridian: Imaginary line in the sky running south to north and passing directly overhead (zenith).

Messier object: French comet hunter Charles Messier developed and published a catalog of non-cometary objects to prevent other comet hunters from mistaking one for a comet. What a guy!

What’s in the Sky?

July 18-22; an hour before sunrise; southeast: The planetary lineup gets company – a waning gibbous Moon gets happy with Jupiter and then Mars.